Hox genes are a group of 39 genes in four clusters that are key transcription factors in specifying segmental identity. The expression pattern of these genes is unusual in that it correlates temporally and spatially with their position within each cluster, termed "colinearity". This could be the result of elements that have been identified within the cluster that, when isolated, have been shown to recapitulate much of Hox gene expression patterns; however, the location of a gene in the cluster still appears to influence its expression characteristics. In order to understand these phenomena more fully we have inserted transposons into bacterial artificial chromosomes (BACs) covering large portions of the murine Hoxb cluster in an effort to determine the 3' extent of elements regulating Hoxb expression. Subsequently, we are scanning these regions for evidences of regulatory function. Once identified, the in-vivo function of these elements will be interrogated by examining Hox marker gene expression from BACs in which these elements have been deleted. These experiments should provide substantial new insights into Hox gene regulation and consequently improve our understanding of segmental identity and development.